As can be seen from the chart, the codec frequency response is flat between 300 and 3,300
Hz. With the high pass filter on, the 3dB points are at 150Hz and 3,600 Hz respectively. With
the high pass filter off, the 3dB point goes down to approximately 15Hz.
5.2
Bluetooth Link Frequency Response
The Codec 13bit linear data is coded within the
Bluetooth
chip using CVSD (Continuous
Variable Slope Decode) encoding for transport over the
Bluetooth
link. CVSD is essentially a
form of Adaptive Differential PCM (ADPCM) and is well suited for voice transmission. It is
forgiving of individual bit corruption as each bit only implements an up or a down shift
relative to the previous level (corruption of the MSB of a 13 bit sample would create a much
larger error term than is possible with ADPCM). A draw back of ADPCM is that it cannot track
large delta changes in signal quickly enough. For voice, this does not present a problem.
The chart below shows the frequency response of the
Bluetooth
link at different levels of
input sine wave.
Over the Air Frequency Transfer Characteristics
0
0.2
0.4
0.6
0.8
1
1.2
0
1000
2000
3000
4000
Frequency (Hz)
V
o
lt
ag
e (
V
)
Codec Loop Around Test
1V Sine W ave
0.6V Sine Wave
0.3V Sine Wave
As can be seen, the frequency response can only be considered to be flat when the input
voltage level is less than a 0.3V peak to peak sine wave.
APN_06002_1v03 Winbond Audio Board Application Note
8